MHT-CET Chemistry · Halogen Derivatives of Alkanes

Classification, Nomenclature and Physical Properties

Halides are classed by the carbon that carries the halogen — sp³ (alkyl, allylic, benzylic), sp² (vinylic, aryl) or sp (alkynyl); boiling point rises with the mass of the halogen and the number of halogens, bond strength falls from C–F to C–I, and a carbon with four different groups makes the molecule chiral.

All Halogen Derivatives of Alkanes notes MHT-CET Chemistry strategy

Why this matters

34 PYQs, 1 HARD — one of the two largest pages and the most predictable. Of the twenty-seven set from 2021 to 2025, eleven ask which formula or name is vinylic, allylic or benzylic (or is NOT); eight ask the highest or lowest boiling point among the halomethanes or the strongest C–X bond; eight ask which halide is chiral or how many chiral carbons it has. Three cards, no calculation.

Concept 1 of 3: Alkyl, Allylic, Benzylic, Vinylic and Aryl Halides

Look at the carbon holding the halogen. sp³ and next to nothing special — alkyl. sp³ and NEXT TO a C=C — allylic. sp³ and attached to a benzene ring — benzylic. sp² in a C=C — vinylic. sp² in the ring itself — aryl. sp in a C≡C — alkynyl. One carbon further away, and the label is lost.

Definition

Vinylic: X on an sp² carbon of a C=C — $\text{CH}_3\text{-CH=CH-X}$ ; a haloalkene.
Allylic: X on the sp³ carbon ADJACENT to a C=C — $\text{CH}_2\text{=CH-CH}_2\text{X}$ , 3-bromopropene, $\text{CH}_3\text{CH=CH-CH}_2\text{X}$ . NOT $\text{CH}_3\text{CH=CH-CH}_2\text{CH}_2\text{X}$ (two carbons away).
Benzylic: X on the carbon directly bonded to the ring — $\text{C}_6\text{H}_5\text{CH}_2\text{X}$ (bromophenylmethane), $\text{C}_6\text{H}_5\text{CHBrCH}_3$ , $\text{C}_6\text{H}_5\text{CBr(CH}_3)_2$ . NOT bromobenzene or 4-bromotoluene (aryl), NOT 1-bromo-2-phenylethane or 1-bromo-2-phenylbutane (X one carbon out).
Aryl: X on the ring carbon — chlorobenzene. Alkynyl: X on an sp carbon — a haloalkyne.
Primary, secondary, tertiary alkyl halides by how many carbons the C–X carbon carries.

Class by the carbon bearing X

\text{sp}^3:\ \text{alkyl / allylic (next to C=C) / benzylic (on ring carbon)};\quad \text{sp}^2:\ \text{vinylic / aryl};\quad \text{sp}:\ \text{alkynyl}

Worked example

Classify:

\text{CH}_2\text{=CH-Cl}

\text{C}_6\text{H}_5\text{-CH}_2\text{CH}_2\text{Br}

\text{CH}_3\text{CH=CH-CH}_2\text{Cl}

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 10th May Shift 2 · Q91Easy

Example 1 · Halogen Derivatives of Alkanes · Classification, Nomenclature and Physical Properties

Which among the following is vinylic halide?

Calling 1-bromo-2-phenylethane benzylic

The bromine carbon is bonded to CH₂, not to the ring. Benzylic needs X on the ring-attached carbon: PhCH₂Br, PhCHBrR, PhCBrR₂.

Drill 10 more on alkyl, allylic, benzylic, vinylic and aryl halides

Concept 2 of 3: Boiling Point and C–X Bond Strength

Boiling point follows van der Waals forces, which grow with molecular mass and surface: heavier halogen, more halogens, longer chain — higher. Bond strength runs the other way: the small fluorine overlaps best, so C–F is the strongest and C–I the weakest bond.

Definition

Same alkyl: $\text{CH}_3\text{I} > \text{CH}_3\text{Br} > \text{CH}_3\text{Cl} > \text{CH}_3\text{F}$ in boiling point — fluoromethane lowest, iodomethane highest.
More halogens: $\text{CHBr}_3 > \text{CH}_2\text{Br}_2 > \text{CH}_3\text{Br} > \text{CH}_3\text{Cl}$ .
Among isomers, branching LOWERS boiling point (less surface).
Bond enthalpy: $\text{C–F} > \text{C–Cl} > \text{C–Br} > \text{C–I}$ ; reactivity in substitution is the reverse (iodides fastest).
Haloalkanes are denser than water for Br and I; slightly polar, insoluble in water.

Two opposite orders

\text{b.p.: } \text{RI} > \text{RBr} > \text{RCl} > \text{RF};\qquad \text{bond strength: } \text{C–F} > \text{C–Cl} > \text{C–Br} > \text{C–I}

Worked example

Arrange bromoethane, 1-bromopropane, 2-bromopropane and iodoethane by boiling point.

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 4th May Shift 1 · Q96Easy

Example 2 · Halogen Derivatives of Alkanes · Classification, Nomenclature and Physical Properties

Which of the following is correct decreasing order of boiling point of compounds?

Using bond strength to rank boiling points

The strong C–F bond does not make CH₃F hard to boil — boiling breaks intermolecular forces, not bonds. Fluoromethane has the LOWEST boiling point and the STRONGEST bond.

Drill 7 more on boiling point and c–x bond strength

Concept 3 of 3: Chiral Haloalkanes and Naming From a Structure

A halide is chiral when the carbon carrying X — or any other carbon — has four different groups. The exam's recurring quartet is 2-bromopropane (two CH₃), 2-bromo-2-methylbutane (two CH₃), 3-bromopentane (two ethyls) and 2-bromo-3-methylbutane (H, Br, CH₃, isopropyl — chiral). Symmetric diiodides such as 1,4-diiodobutane are inactive.

Definition

Chiral: 2-bromo-3-methylbutane, 2-iodo-3-methylbutane, sec-butyl bromide (2-bromobutane), 2-iodopentane, 3-iodohexane.
Achiral: 2-bromopropane, 2-bromo-2-methylbutane, 2-iodo-2-methylbutane, 3-bromopentane, 3-iodopentane, n-, iso- and tert-butyl bromide, 1,4-diiodobutane (plane of symmetry).
Counting: 2-chloro-3,4-dimethylhexane has THREE chiral carbons (C2, C3, C4); 3,4-dibromohexane two; 1,2- and 1,3-diiodobutane one each; 2,3-diiodobutane two.
Naming a drawn halide: longest chain through the double bond, lowest locant to C=C — $\text{CH}_3\text{CH=CH-C(CH}_3)(\text{Br)-CH}_3$ is 4-bromo-4-methylpent-2-ene.

Chirality test

\text{C}abcd,\ a \neq b \neq c \neq d \Rightarrow \text{chiral; two identical groups} \Rightarrow \text{achiral}

Worked example

How many chiral carbons in 2,3-dibromopentane and in 2-bromo-2-chlorobutane?

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 15th May Shift 1 · Q98Moderate

Example 3 · Halogen Derivatives of Alkanes · Classification, Nomenclature and Physical Properties

Which of the following isomers of C

_4

_9

Br is a chiral molecule?

Checking only the carbon that carries the halogen

In 2-chloro-3,4-dimethylhexane C3 and C4 are also stereocentres. Walk every branch-point carbon before you count.

Drill 7 more on chiral haloalkanes and naming from a structure

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (3)

Alkyl, Allylic, Benzylic, Vinylic and Aryl Halides
Class by the carbon bearing X
$\text{sp}^3:\ \text{alkyl / allylic (next to C=C) / benzylic (on ring carbon)};\quad \text{sp}^2:\ \text{vinylic / aryl};\quad \text{sp}:\ \text{alkynyl}$
Boiling Point and C–X Bond Strength
Two opposite orders
$\text{b.p.: } \text{RI} > \text{RBr} > \text{RCl} > \text{RF};\qquad \text{bond strength: } \text{C–F} > \text{C–Cl} > \text{C–Br} > \text{C–I}$
Chiral Haloalkanes and Naming From a Structure
Chirality test
$\text{C}abcd,\ a \neq b \neq c \neq d \Rightarrow \text{chiral; two identical groups} \Rightarrow \text{achiral}$

Watch out for (3)

Calling 1-bromo-2-phenylethane benzylic
→ Alkyl, Allylic, Benzylic, Vinylic and Aryl Halides
Using bond strength to rank boiling points
→ Boiling Point and C–X Bond Strength
Checking only the carbon that carries the halogen
→ Chiral Haloalkanes and Naming From a Structure

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